The attempt to counterfeit valuable documents, especially notes, is a fraudulent activity that is as old as the existence itself of said documents.
All those elements contributing to the authentication of the document in question or which makes their counterfeiting as difficult as possible are included in the security element concept. Said security elements basically consist of a particular coating in the surface of the substrate or an element comprised or applied to the surface of the substrate.
The use of special printing processes providing the security documents with a relief that can be perceived by touch, such as the one incorporated in some elements of the current Euro notes of different denominations is known. These tactile marks further fulfill the purpose of aiding the blind in identifying said documents.
In contrast to the previous method, the incorporation of security marks formed by holes is also known, which marks, like the previous ones, have the advantage of being easily verifiable without the need for special equipment. Document ES 2 145 486 T3 describes a method for carrying out marks of this type by using laser light, a method by which holes are obtained having a diameter such that they are invisible when looked at in reflection whereas they are visible in plain sight when looked at in transmission, i.e., when looking at them against a light source.
Security documents, especially banknotes, frequently incorporate optically variable devices such as security threads, holograms, layers of interference elements or liquid crystal materials providing different color effects depending on the viewing angle, making counterfeiting the document by photocopying it extremely difficult.
A usual security element in valuable documents is the incorporation of the so-called watermarks, representing a figure or group of characters that can only be seen when the document is looked at against the light. The manufacture of papers incorporating this type of marks is carried out in paper manufacturing machines with a perforated cylindrical drum, therefore the elevation or areas impermeable to water reduce the depositing of fibers on the surface of said drum and therefore the sheet of paper thus manufactured has locally different thicknesses.
As regards the incorporation of holograms in security documents, the usual method consists of manufacturing the so-called main holograms and from them casting the holograms required per copy in the press molds, thus obtaining large amounts of holograms. Cast holograms are normally manufactured as multi-layer elements on a separate carrier, such that they are joined to the document in question by means of a layer of adhesive material. This manufacturing method has deficiencies with respect to document security because it is possible to remove the hologram from the document by heating the adhesive material and therefore, transferring said hologram to a counterfeit document, adhering it thereto.
Document EP 0 338 378 B1 describes a process for manufacturing security documents including holograms solving the previous drawback, because in this case the hologram is cast directly on the material forming the document, first printing the paper on both sides and subsequently providing said paper with a holographic structure in certain areas. The process consists of simultaneously transferring to the paper forming the security document both the casting resin and the hologram relief structure, coating the surface structure of the mold of the press with a curable resin. Once the mold and the paper come into contact, the resin adheres to the surface of the paper after it has been cured, having a holographic structure in relief. A thin metallized layer allowing observation by reflection of the holographic information is subsequently vacuum-applied. The drawback of this manufacturing process lies in the need to cure the resin by means of electron radiation, which curing is complex, damaging for the paper and involves high costs.
In view of the foregoing, the object of the present invention is to develop paper for forming security documents allowing the incorporation of several security elements that cannot be easily detached from said documents and which prevent their counterfeiting to a considerable extent, it also being desirable that the obtained paper has homogenous surfaces and is stable for all purposes.